In the manufacture of glass fiber mats, typically glass fiber is spun from molten glass and is sprayed with uncured binder compound as the fiber is being showered onto a moving chain conveyor. The conveyor carries the resulting glass fiber blanket through curing ovens wherein the binder material is exposed to elevated temperatures for an appropriate time period to complete the curing of the binder. After its exit from the ovens, the mat or blanket is cooled by a stream of air from a fan, and certain of its properties may be measured, typically with radiation gauges.
The mass per unit area of the traveling mat has been measured, with various degrees of success, using beta ray gauges, gamma ray or X-ray gauges, or infrared radiation gauges using combinations of wavelengths. One special example disclosed in our U.S. Pat. No. 3,809,903 uses gamma rays from the americium-241 isotope to generate substantially monochromatic X-rays in the range of about 14 to 30 kev, which in turn are used in the mass per unit area measurement. Very recently, curium-244 sources have become available, and commercially successful gauges for glass fiber mats have made use of the substantially monochromatic X-rays, in the range of about 14 to 21 kev, from this isotope as such for the mass per unit area measurements. The mass per unit area measurements have been used to automatically control the speed of the chain conveyor, thereby determining the amount of coated glass fibers deposited while a section of the conveyor moves through the felting chamber, with the objective of maintaining the weight per unit area of the mat constant along its length.
Various attempts have been made to measure the mass of the binder material per se, for example by taking advantage of the fact that glass is substantially transparent to certain optical (e.g., infrared) and X-ray wavelengths that are more or less attenuated by the binder materials. The objective of this measurement is to be able to control the mass of the binder by regulating the amount or the dilution of the spray material applied.
In the Sturm application No. 431,179 supra, it is proposed to measure the degree of cure of the resinous binder which has been effected by its exposure to the elevated temperatures in the curing ovens, and to automatically control the oven temperatures in response to the cure measurement. The cure measurement utilizes infrared radiation absorption, which is influenced somewhat by scattering effects in the thick glass fiber mat and by the spectral effects of certain constituents such as iron oxide that are accidentally present in variable unknown amounts in the glass from which the fibers are spun. These extraneous scattering and spectral effects on the infrared measurements are compensated by the use of a mass per unit area signal derived independently from a gamma ray or isotopic X-ray gauge. The computation of cure involves a set of variable values representing the mass per unit area of the binder material per se in the glass fiber mat, which values are obtained basically from the infrared absorption measurements. However, in the case of very heavy glass fiber mats (large mass per unit area) the infrared radiation has insufficient penetrating power to provide the binder mass information.
Hence there has been a need for an improved instrument capable of measuring quantitatively and reliably the mass of resinous binder per unit area of heavy glass fiber mats, which may be of the kind referred to as high-density mats and are used, for example, as heat insulation for refrigerator doors or as sound-deadening and heat-insulating mats under automobile hoods. Such mats typically range in thickness from about one-half to five inches (1.3 to 13 centimeters) and in mass per unit area from about 3000-8000 grams per square meter, of which about six to fourteen percent is binder, made up of cured components of urea formaldehyde and phenol formaldehyde. The "binder weight" measurement is difficult because of the macrostructure and microstructure of the mat and by the relatively small amount of binder as compared to the weight of the glass fibers. Moreover, while the effective atomic numbers of the glass and the binder are sufficiently different to provide a workable contrast for the purposes of more ordinary X-ray or gamma-ray measurements, they are not markedly different.